Andrej Gajdos, Developer in Prague, Czech Republic
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Andrej Gajdos

Verified Expert  in Engineering

Full-stack Developer

Prague, Czech Republic
Toptal Member Since
August 31, 2016

Andrej is a full-stack developer with a focus on the PERN/MERN stack and over 10 years of experience delivering software. As a fractional CTO and tech lead, Andrej has managed several teams delivering software for worldwide clients in many industries, including healthcare, finance, and eCommerce. Thus far, Andrej has delivered over 20 projects for startups, digital agencies, and Fortune 500 companies such as Apple and Binance.


JavaScript, React, Node.js, MongoDB, PostgreSQL, MySQL, Websites, Development...
Node.js, Heroku, MongoDB, React, Server-side Rendering (SSR), Optimization...
Amazon Web Services (AWS), Amazon EC2, REST APIs, Databases, React, Node.js...




Preferred Environment

OS X, Visual Studio Code (VS Code)

The most amazing...

...thing I've developed is a pixel-perfect user interface for Apple.

Work Experience

Fractional CTO, Software Architect, Senior Full-stack Developer

2017 - PRESENT
  • Consulted strategically with companies on the product development roadmap, developing workflow, setting project infrastructure, architecting and implementing third-party integrations, and providing code reviews.
  • Mentored senior developers with best practices, design patterns, and tools to scale web applications in maintainability and performance.
  • Refactored and improved codebases that saved customers at least six figures in future development.
  • Developed an interactive data visualization that renders thousands of buildings on a map, and users can edit buildings by editing polygons.
Technologies: JavaScript, React, Node.js, MongoDB, PostgreSQL, MySQL, Websites, Development, Front-end, APIs, CTO, Architecture, CSS, HTML, CSS3, HTML5, Next.js, Express.js, Tailwind CSS, D3.js, MobX, Responsive UI, UI Development, Single-page Applications (SPA), Web Development, Full-stack Development, Front-end Architecture, Front-end Development, Data Visualization, Storybook, Amazon Web Services (AWS), CircleCI, Contentful, Amazon EC2, Heroku, Technical Leadership, Solution Architecture, Full-stack, Minimum Viable Product (MVP), API Design, TypeScript, REST APIs, User Interface (UI), Figma, Software Engineering, Agile, SQL, Web Applications, Software Architecture, Interactive UI, Git, Responsive Design, GraphQL, Unit Testing, Dashboards, Databases, API Integration, User Experience (UX), MERN Stack, Leadership, Startups, Crypto

Software Architect

2022 - 2023
  • Optimized the platform and scaled infrastructure to allow the users' growth from 50,000 to 1 million.
  • Solved performance issues and user flows from 10-15 seconds to 1.5 seconds.
  • Improved database search functionality to allow users to search across millions of records within 20 seconds.
Technologies: Node.js, Heroku, MongoDB, React, Server-side Rendering (SSR), Optimization, Datadog, Sentry, Amazon, MongoDB Atlas, MERN Stack, Apollo, GraphQL, Refactoring, Leadership, Webpack, TypeScript, Express.js

Fractional CTO

2022 - 2022
  • Provided strategic consulting on a product development roadmap, setting engineering pacing to successfully finish an MVP.
  • Set up infrastructure and CI pipelines, and conducted code reviews to identify issues before they translated into technical debt.
  • Helped developers with best practices and leveraged tooling to speed up the development process and ensure code quality.
Technologies: Amazon Web Services (AWS), Amazon EC2, REST APIs, Databases, React, Node.js, CTO, Architecture, Technical Leadership, Solution Architecture, Minimum Viable Product (MVP), Software Architecture, CI/CD Pipelines, Leadership, Startups, Express.js

React and Gatsby Developer

2018 - 2019
  • Built customizable UI components used across multiple projects. Built funds management and payment systems in the exchange platform.
  • Worked on the Nash extension, a browser-based cryptocurrency wallet, and Chrome extension.
  • Built the company website in React, Gatsby, and TypeScript.
Technologies: D3.js, TypeScript, Styled-components, GraphQL, Gatsby, React, JavaScript, Websites, Development, Front-end, HTML, CSS, UI Development, Front-end Architecture, Front-end Development, Single-page Applications (SPA), Storybook, Minimum Viable Product (MVP), User Interface (UI), Web Applications, Interactive UI, Unit Testing, Git, Dashboards, User Experience (UX), Startups, Crypto

React Developer

2018 - 2018
  • Designed and built a functional prototype of an event management platform for organizing swimming teams and swimming competitions.
  • Designed a new user interface based on the customer's prototypes.
  • Built a functional prototype from scratch as a single-page application.
Technologies: Redux-Saga, React, JavaScript, Front-end, CSS, HTML, Front-end Architecture, Front-end Development, Single-page Applications (SPA), Prototyping, Minimum Viable Product (MVP), User Interface (UI), Web Applications, Interactive UI, Dashboards

React Developer

2018 - 2018
  • Helped deliver a React Native project used for managing fitness studios.
  • Fixed various bugs and issues in the React Native project.
  • Optimized the React Native project for the Android platform.
Technologies: Redux, React Native, JavaScript, User Interface (UI), Startups

Principal Software Engineer

2017 - 2018
  • Worked as a full-stack JavaScript developer using React and Node.js.
  • Designed and built an interactive podcast platform. Podcasters can upload audio files and add "spots" to specific moments in their podcasts, which can be anything from links to social media posts, videos, or images.
  • Built a single-page app from scratch with a complex user interface, including user interactions and animations.
  • Architected and built the project architecture and server.
Technologies: InVision, X (formerly Twitter) API, Babel, Webpack, SVG, D3.js, Redux, React, Knex.js, Objection.js, Koa, Node.js, JavaScript, PostgreSQL, Amazon S3 (AWS S3), Heroku, Development, APIs, Single-page Applications (SPA), Full-stack Development, Front-end Development, Front-end Architecture, CTO, Technical Leadership, Minimum Viable Product (MVP), Full-stack, Back-end, Back-end Development, HTML, CSS, API Design, REST APIs, User Interface (UI), Software Engineering, SQL, Web Applications, Software Architecture, Architecture, Interactive UI, Databases, API Integration, User Experience (UX)

Pre-sales Engineer

2016 - 2017
  • Presented and demonstrated products to potential customers from all over the world.
  • Demonstrated product features to developer teams and showed how to use them in the development process with a specific workflow, environment, and toolset.
  • Helped sales associates close deals by presenting our products in specific use cases.
  • Trained sales associates on how to effectively present and sell our tools to developer teams and managers.
  • Assisted customers with installation, troubleshooting, and other customer services for configuration issues.
Technologies: Java, Consulting, IT Consulting, Sales, Tech Sales, Technical Consulting

Software Engineer

2015 - 2016
  • Worked mainly as a front-end developer on dashboards and data visualizations.
  • Implemented new features and fixed bugs in enterprise applications used for data quality management and rewrote UI components to modern technologies.
  • Designed and implemented the front end of an interactive application for customer data management.
  • Designed and implemented the server-to-sever communication and a server push in the GWT application.
Technologies: Errai, ActiveMQ, JMS, JSTL, Jakarta Server Pages (JSP), D3.js, Babel, Gulp, Webpack, SCSS, Bootstrap, jQuery, Backbone.js, Immutable.js, Flux, React, Emacs, JavaScript, Java, APIs, HTML, CSS, User Interface (UI), REST APIs, Software Engineering, Web Applications

Data Visualization Developer

2013 - 2015
Techniserv, Taranzo
  • Analyzed, designed, and implemented solutions for gathering and processing structured and unstructured data from social networks and various other sources.
  • Designed and implemented a module for building and visualizing a social network.
  • Built data processing from social networks, such as YouTube and Twitter.
Technologies: JUNG, X (formerly Twitter) API, YouTube API, OrientDB, D3.js, Bootstrap, jQuery, JSTL, Jakarta Server Pages (JSP), Apache Maven, Spring MVC, Liferay, JavaScript, Java, APIs, Full-stack Development, Front-end Development, Software Architecture, Architecture, Back-end Development, Interactive UI, API Integration

Software Tester

2012 - 2013
Amberg Group
  • Executed both manual and automated tests and analyzed the results.
  • Developed and maintained the whole test infrastructure with automated tests.
  • Designed test cases, wrote documentation, and reported issues.
Technologies: Ruby, Python, JavaScript, Ruby on Rails (RoR), Selenium

Helped the team as a fractional CTO with strategic consulting on the product development roadmap. Managed and led a team of developers. I identified issues via code reviews before they were translated into technical debt, helped developers with best practices, and leveraged tooling to speed up the development process and ensure code quality. I also architected and built the project's infrastructure, CI pipeline, and third-party integrations.

GoSchoolBox is a SaaS platform that serves as the technical infrastructure layer between school districts and tutoring companies. The number of users grew from 50,000 to 1 million. The GoSchoolBox platform has suffered many application stability issues, such as spontaneous error messages, gateway timeout errors, and slow page load times. As a software architect, I ensured platform stability, resiliency, and scalability. I helped the team to integrate and set up APM with metrics and performance dashboards to detect and diagnose end-users' experience with mission-critical services. I refactored and fixed critical user flows to improve user flow performance, server throughput, and page load performance. I optimized the MongoDB search performance to allow users to search millions of records within seconds.

An investment community platform for investors to discover deals and identify co-investors. The platform allows investors to interact with each other, keep track of deals, and create a virtual investment club for a curated group of private investors.

I designed and implemented the integration with 3rd-party services, such as DocuSign, Slack, and Stripe. I also helped the team set up the project tooling, fix various issues, and implement new features.

Stackbit is a website builder that helps developers of any level build their Jamstack sites based on Next.js and Tailwind fast.

I helped Stackbit's team set up the tooling and design the architecture for the design system of the main platform. I assisted and mentored senior React developers on best practices and patterns to solve complex flows in React projects. I fixed various issues and bugs in the platform and on additional feature sets for enterprise clients.

Getctrl is a customer data platform for customer success professionals. It empowers post-sales people to bring data from different apps to create workspaces.

I developed new features and fixed the existing codebase as a front-end developer. I also helped and mentored senior React developers on best practices.
A blazing-fast static website built with React, Gatsby, TypeScript, and style components. All pages are fully responsive and optimized for mobile viewing.

I built this company website from scratch, adding and reusing existing components from Storybook.

Cryptocurrency Exchange Platform
A cryptocurrency exchange platform.

Joined the project as a React developer and implemented a design system, portfolio management, and payment systems. I used React, TypeScript, Storybook, and GraphQL.

Archipelago is a next-generation NFT marketplace that offers curated generative art NFTs. As a front-end consultant, I helped the team review the existing codebase, refactor problematic flows, and fix various bugs and issues.

Security Tokens Offering Platform

A secure, customer-centric, decentralized online securities exchange marketplace allows investors at all levels to buy and sell digital securities. The customer is a leading cryptocurrency exchange. I worked as a senior front-end engineer. I operated as an architect for front-end implementation and reviewed and audited the code of other front-end developers.

System Master Planner

A computational tool for master planners, energy and resource managers, and policymakers. It allows users to evaluate sustainability and resiliency scenarios for large buildings and other assets.

I worked as a React developer and developed interactive data visualization, rendering thousands of buildings as polygons on a LeafletJS map.

An event management platform for managing swimming teams and swimming competitions. I designed a new user interface based on customers' prototypes and built a functional prototype.

Development tools: JavaScript, React, and Redux-Saga.

Saycaster Rebranded to Bitcast

An interactive podcast platform with the following features:
• Podcasters can upload their audio and add 5-20 spots to their podcasts.
• These spots can be anything from links to social media posts to other videos or images.
• Listeners can listen and comment on these specific spots.

SushiSwap Migrator
A Web3.js project where users can migrate LP positions from Uniswap to SushiSwap. Users can interact with the Ethereum blockchain through MetaMask, view the ERC-20 tokens list and LP positions, and add new ERC-20 tokens.


A project for a company in the marine industry. I worked as a data visualization specialist to build an interactive visualization of a Neo4j graph database. I developed this visualization in React, D3.js, and Storybook.


I built the user interface for this eCommerce tool in React. It helped the CEO and sales associates to maintain and process details about potential customers. I built the user interface using React and Tailwind.

Notion API Connector
I built this Google Chrome extension to connect and import data from any API to Notion.


A Chrome browser extension that integrates data between Salesforce and Notion. It allows users to pull Salesforce data directly into the Notion workspace.



Filipi is a furniture eCommerce store built with Next.js, Node.js, and MySQL. I helped the owner migrate the whole solution to another server, fix various issues and bugs in SEO, manage data, and set up analytics tools.


Created a Node.js back-end service that processes data from 3rd-party APIs like 3rd-party exchanges, GitHub API, Twitter API, and Telegram API. The server is deployed on AWS. I helped the client fix the project after previous developers and replaced the headless CMS solution.

Software Development Blog
My freelance software development blog. It was built with WordPress and JavaScript. I blog about tips, tutorials, and technical solutions in development in JavaScript, React, Node.js, and related technologies.

Ivana Neckarova

A personal WordPress website, where I customized the template to achieve unique design and responsiveness for mobile devices. I set up WordPress plugins to improve website performance and search engine optimization.

Link Preview Generator
Get a URL's preview data (title, description, image, and domain name). I built this Node.js library that uses a Puppeteer headless browser to scrape the website and get the website's title, description, image, and others.

Neo Transfer
Neo Transfer allows you to log in with a private key, see balance, and transfer NEO or GAS within Testnet. The project is based on the JavaScript SDK for the NEO blockchain. The front end is implemented in React.

Newsfeed Cryptogoods
Newsfeed for events on crypto goods. A single, responsive page for the newsfeed. Polling occurs every 15 seconds for new events to update the newsfeed in real-time. This front-end project was built using JavaScript, React, Redux, Redux-Saga, and Twitter Bootstrap.

Authenticating Users in SPA using Node.js, Passport.js, React, and Redux
User authentication in a single-page application with Node.js, React, Redux, and Koa combined with Passport.

Development tools: JavaScript, Node.js, React, Redux, Passport.js.

Nested Datatables
jQuery DataTables plugin for rendering nested data tables in rows. Inner tables are independent of the data and layout from the outer table. I created this plugin for a data management product.

Development tools: JavaScript and jQuery.

Webpack Setup for ES6, React, Sass, and Bootstrap
Webpack configuration to bundle ES6, React, Sass, and Bootstrap for production and development.

Development tools: JavaScript, React, and Webpack.

Social Graph

A Liferay portlet to visualize a social network as a directed multigraph. Jung library is used for building the multigraph, which is stored in OrientDB. Visualization and other components are built in D3.js and Bootstrap.

Development tools: Java, JavaScript, Liferay, JSP, Jung, OrientDB, D3.js, Bootstrap

Sample Source Code

This source code is the front end of the application for customer data management. I developed everything but the login page and used technologies such as ES6 (Babel), React, Immutable.js, Bootstrap table, and webpack.


• Install Node.js from
• Install Bower from
• Go to the project root directory in the terminal and install other dependencies, i.e., npm install and Bower install.
• Follow npm run start.
• Open the URL localhost:8080/ and type in:
a) User: andrej
b) Password: nachos

SVG Sprite into PNG
A workflow converts SVG sprite into PNG images utilizing Gulp.

Development tools: JavaScript, Gulp

Server Push in GWT Application with Errai
Implementation of server push messages from server to client in GWT application with Errai framework.

Development tools: Java, GWT, Errai

Contract Generator Tool

Some insurance brokers have a list of insurances and clients in XLS format and need to generate various contracts based on a contract template in DOC and DOCx format. The user can set up a database (XLS or XLSx) and required columns, which will be parsed into the contract. The application has a database of insurance in SQLite. The main window contains a list of clients with detailed information, where the user can instantly search clients, add required information (date, overpayment, insurance from database), choose contract templates, and generate a contract (DOC or DOCx) for the client. The UI is built in WPF and ADO.NET is used for parsing data from a Microsoft Excel file.

Development tools: C#/.NET, WPF, SQLite, ADO.NET

Personal Calendar
A basic personal calendar application with two view modes (week/month). The application generates log files and uses a MySQL database. The presentation layer is built in Swing and SwingWorker. Connections to the database are developed in JDBC. The project contains simple JSP pages for viewing a list of events and their detailed information.

Development tools: Java, JSP, Swing, SwingWorker, MySQL, JDBC
2013 - 2015

Master's Degree in Service Science, Management, and Engineering

Masaryk University Brno - Brno

2014 - 2014

Erasmus Exchange Student in Master in Innovation and Research in Informatics

Polytechnic University of Catalonia - Barcelona

2009 - 2013

Bachelor's Degree in Applied Informatics

Masaryk University - Brno


Underscore.js, React, Node.js, React Router, REST APIs, Atomic CSS, Passport.js, jQuery Plugins, jQuery UI, Redux-Saga, D3.js, Immutable.js, JSTL, Chai, jQuery, JavaScript Charting, AWS Amplify, GitHub API, X (formerly Twitter) API, YouTube API, Objection.js, JDBC, Backbone.js, OpenGL, Win32 API, Windows Forms (WinForms), JMS, Leaflet, MobX, Web3.js, Stripe, Stripe API, Puppeteer, Slack API, Salesforce API


Git, Redux Thunk, Webpack, Gulp, Bower, Mocha, Emacs, PostCSS, Knex.js, Babel, GitLab, GitHub Pages, GitHub, Trello, InVision, ActiveMQ, Apache Maven, CircleCI, Figma, LaunchDarkly, Slack, Notion, Sentry, MongoDB Atlas, NPM


Express.js, Redux, Next.js, Koa, Bootstrap, Spring MVC, Flux, JUnit, Tailwind CSS, Material UI, Errai, React Native, Ruby on Rails (RoR), Jakarta Server Pages (JSP), Swing, Windows Presentation Foundation (WPF), .NET, Jest, Cypress, Selenium


JavaScript 6, CSS, HTML, CSS3, HTML5, JavaScript, TypeScript, Object-oriented JavaScript (OOJS), Java, Less, SCSS, Sass, ECMAScript (ES6), SQL, GraphQL, C, Python, Ruby, PHP, C#, UML, C++


Model View Controller (MVC), Unit Testing, Object-relational Mapping (ORM), Agile Software Development, Functional Programming, REST, Object-oriented Programming (OOP), Object-oriented Design (OOD), Software Testing, Automated Testing, Search Engine Optimization (SEO), Requirements Analysis, Web Architecture, Responsive Layout, Agile, Refactoring


JSON, PostgreSQL, Amazon S3 (AWS S3), Graph Databases, SQLite, MySQL, OrientDB, MongoDB, Relational Databases, Databases, Datadog, Cloud Firestore


Firebase, Java EE, WordPress, Contentful, Amazon Lightsail, AWS Lambda, Amazon Web Services (AWS), OS X, Heroku, Amazon, MacOS, Windows, Linux, Blockchain, X (formerly Twitter), Amazon EC2, Web, Mapbox, Visual Studio Code (VS Code), Salesforce, Vercel


Responsive UI, UI Development, Ajax, Front-end, Single-page Applications (SPA), Web Development, Full-stack, Front-end Development, Front-end Architecture, Websites, Development, React Hooks, Web App Development, Minimum Viable Product (MVP), User Interface (UI), Atomic Design, Software Engineering, Web Applications, Interactive UI, Dashboards, UI Components, UI Libraries, Component Libraries, API Integration, APIs, Back-end, ESLint, Technical Consulting, Back-end Development, Writing & Editing, Data Visualization, Presentations, Sales, Customer Service, Consulting, Programming, Software Design, Styled-components, Gatsby, CTO, Architecture, Software Architecture, Full-stack Development, Solution Architecture, Slate, Responsive Design, Apollo, User Experience (UX), MERN Stack, Leadership, Startups, Cloudflare, Prismic, Webhooks, Storybook, JUNG, SVG, Web Scraping, Social Networks Development, Data Analysis, Liferay, BPM, Messaging Patterns, Sales Presentations, Chrome Extensions, Ethers.js, Scraping, Prototyping, Technical Leadership, IT Consulting, Tech Sales, CSS-in-JS, Technology Consulting, Team Leadership, Local SEO, API Design, eCommerce, Strapi, Solution Design, APM, Performance, Server-side Rendering (SSR), UI Testing, CI/CD Pipelines, Optimization, Crypto, Non-fungible Tokens (NFT), SaaS

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